As a compound semiconductor highly sensitive to infrared radiation, CdHgTe (cadmium mercury tellurium) is well known, and infrared detectors utilizing the photoconductivity of CdHgTe and photodiodes utilizing the photovoltaic effect of CdHgTe have been developed.
CdHgTe is generally frown on a CdTe substrate or a Cd.sub.x Zn.sub.1-x Te(x=0.97.about.0.98) substrate by LPE (Liquid Phase Epitaxy) or MOCVD (Metal Organic Chemical Vapor Deposition). Since the CdHgTe crystal includes Hg which has a very high vapor pressure, Hg is likely to be lost by dissociation during crystal growth, causing vacancies. Since the vacancies act as acceptors, the material grown by, for example, LPE is p type CdHgTe having a carrier concentration of 10.sup.16 .about.10.sup.17 cm.sup.-3.
The photoconductivity of the p type CdHgTe, i.e., the variation of the electric conductivity with incident light is not so high, so that it is difficult to fabricate a photoconductive detector using the p type CdHgTe obtained by LPE. Therefore, after the crystal growth, Hg is diffused into the p type CdHgTe layer, whereby the p type CdHgTe layer is converted to n type which exhibits a higher photoconductivity.
FIGS. 5(a) to 5(c) are views for explaining a conventional Hg diffusion method disclosed in Japanese Patent Published Application No. 62-34157. FIG. 5(a) schematically shows a quartz ampoule 51 in which a sample 50 and Hg 61 are sealed. The sample 50 and the Hg 61 are separated from each other by the narrow part 52 of the quartz ampoule. FIG. 5(b) shows the sample 50 in detail, in which a CdHgTe crystal layer 101 having a thickness of 10.about.20 microns is grown by LPE on a CdTe substrate 100 having a thickness of several hundreds of microns. FIG. 5(c) is a graph showing the temperature profile during the mercury diffusion.
The sample 50 shown in FIG. 5(b) and the Hg 61 are sealed up in the quartz ampoule 51 and annealed at the temperatures shown in FIG. 5(c) for 20 hours. During the annealing, Hg partial pressure from the Hg 61 is applied to the CdHgTe layer 101 of the sample 50 and then Hg atoms diffuse into the CdHgTe layer 101 and fill the Hg vacancies in that layer, whereby the p type CdHgTe layer 101 is converted to n type. A photoconductive detector is produced using the n type CdHgTe crystal layer thus formed.
As shown in the temperature profile of FIG. 5(c), the annealing is carried out with the temperature of the sample 50 kept higher than the temperature of the Hg reservoir to avoid the Hg from condensing on the surface of the CdHgTe layer when the temperature is reduced.
FIG. 6 is a cross-sectional view showing the structure of a pixel of a photodiode type infrared detector using a CdHgTe crystal layer as a light-to-electricity conversion layer. In FIG. 6, reference numeral 10 designates a CdTe substrate. A p type CdHgTe crystal layer 11 is disposed on the CdTe substrate 10 and an n type region 41 is formed in the CdHgTe layer 11 by diffusion of Hg. A p-n junction 42 is formed between the p type CdHgTe layer 11 and the n type region 41. A p side electrode 71 is disposed on the p type CdHgTe layer 11 where the n type region 41 is absent and an n side electrode 72 is disposed on the n type region 41. Reference numeral 33 designates an insulating protective film.
FIGS. 7(a) to 7(c) are cross-sectional views showing a method for fabricating the p-n junction 42 of the photodiode type infrared detector shown in FIG. 6. In these figures, reference numeral 34 designates a diffusion mask.
As shown in FIG. 7(a), a p type CdHgTe crystalline layer 11 is formed on the CdTe substrate 10 by LPE or the like. Then, as shown in FIG. 7(b), a diffusion mask 34 comprising such as ZnS and having an aperture opposing a pixel region is formed on the p type CdHgTe layer 11. The wafer shown in FIG. 7(b) is put in a quartz ampoule together with Hg, and Hg atoms are diffused into the CdHgTe crystal in the same way as shown in FIGS. 5(a)-5(c), resulting in an n type CdHgTe region 41 shown in FIG. 7(c) which serves as a pixel.
In the conventional Hg diffusion method, however, it is necessary to seal up the CdHgTe crystal 11 and the Hg 61 in the quartz ampoule, resulting in difficulty in handling. In addition, as shown in FIG. 5(c), the temperature difference between the CdHgTe crystal 11 and the Hg 61 in the quartz ampoule 51 is required. Furthermore, the size of the CdTe substrate and CdHgTe film is restricted to the diameter of the quartz ampoule 51, so that it is not possible to increase the size of the CdHgTe crystal.